THE INTERIOR OF A LEAF IS MADE UP OF MESOPHYLL CELLS

 

   THE TWO TYPES OF MESOPHYLL

   CELLS ARE:

 

   1.   SPONGY

   2.   PALISADE

  

THERE ARE AIR SPACES BETWEEN THE MESOPHYLL CELLS THAT ARE INTERCONNECTED

 

AN AIR SPACE MAY BE LOCATED BENEATH THE STOMATAL APPARATUS.  THIS SPACE IS KNOWN AS THE SUBSTOMATAL CRYPT OR SUBSTOMATAL SPACE

 

 

 

 

 

 

 

 

TRANSPIRATION OCCURS BY A TWO STEP PROCESS

 

   1.   WATER EVAPORATES FROM THE

      SURFACES OF CELLS LINING

      THE SUBSTOMATAL CRYPT

 

   2.   WATER DIFFUSES FROM THE

      SUBSTOMATAL CRYPT THROUGH          THE STOMATAL PORES AND INTO    THE ATMOSPHERE. THIS IS

      KNOWN AS stomatal trans-

      piration.

 

STOMATAL TRANSPIRATION ACCOUNTS

FOR 90 TO 95 PERCENT OF THE WATER LOST FROM LEAVES

 

5 TO 10% OF WATER LOSS IS DUE TO CUTICULAR TRANSPIRATION

 

MEASURING TRANSPIRATION

 

THERE ARE TWO METHODS:

   1.   WEIGHT LOSS

   2.   GAS EXCHANGE

WEIGHT LOSS (LYSIMETER) METHOD

 

   1.   SEAL A WELL WATERED PLANT

      TO PREVENT LOSS OF WATER

      THROUGH THE POT OR FROM THE

      SOIL

 

   2.   WEIGHT PLANT OVER A FIXED

      PERIOD OF TIME

 

   3.   CALCULATE WATER LOSS DUE TO

      TRANSPIRATION

 

GAS EXCHANGE METHOD

 

   1. SEAL LEAF OR BRANCH IN A

      TRANSPARENT CHAMBER WITH A

      FLOWING AIR STREAM

 

   2.   WATER CONTENT IN AIR ENTER-

      ING AND AIR LEAVING THE

      CHAMBER IS MEASURED

 

   3.   THE DIFFERENCE BETWEEN             THESE TWO VALUES IS A

      MEASURE OF TRANSPIRATION

THE DRIVING FORCE OF TRANSPIRATION

 

   *VAPOR PRESSURE GRADIENT*

 

VAPOR PRESSURE IS AFFECTED BY:

 

   1.   NUMBER OF SOLUTE MOLECULES.

      THIS EFFECT IS MINIMAL

      WHY?

 

      EX. 0.5 MOLAL SOLUTION

      CONTAINS 0.5 MOLES OF

      SOLUTE IN 1000 GM (55.5             MOL)OF WATER.

     

      A MOLE FRACTION OF WATER IN

      A O.5 MOLAL SOLUTION IS

      55.5/(55.5 + 0.5) = .991

    

   2.   TEMPERATURE

     

      HAS A GREAT EFFECT ON VAPOR

      PRESSURE BECAUSE IT IN-

      CREASES THE KINETIC ENERGY

      OF WATER MOLECULES.  THE

      FASTER THE WATER MOLECULES       MOVE, THE HIGHER THE VAPOR          PRESSURE.

 

   WATER VAPOR DIFFUSES DOWN A

   VAPOR PRESSURE GRADIENT (FROM

   HIGH TO LOW).  THE VAPOR

   PRESSURE INSIDE A LEAF IS

   GREATER THAN OUTSIDE SO WATER

   MOVES FROM INSIDE TO OUTSIDE

   A PLANT.

 

THE INFLUENCE OF HUMIDITY, TEMPERATURE, AND WIND SPEED ON TRANSPIRATION RATE

 

LEAVES MAY BE SURROUNDED BY A LAYER OF UNSTIRRED AIR CALLED

A BOUNDARY LAYER

 

THE THICKNESS OF THE BOUNDARY

LAYER WILL AFFECT THE RATE OF

TRANSPIRATION.  THE THICKER THE

BOUNDARY LAYER THE MORE RESISTANCE THERE WILL BE FOR TRANSPIRATION TO OCCUR.

 

THE THICKNESS OF THE BOUNDARY

LAYER IS AFFECTED BY WIND SPEED, HAIRINESS OF LEAF, TOPOGRAPHY OF LEAF AND SIZE OF LEAF.

 

INFLUENCE OF HUMIDITY

 

HUMIDITY IS DEFINED AS THE WATER

VAPOR CONTENT IN AIR AND CAN BE

EXPRESSED AS RELATIVE HUMIDITY

(RH)

 

RELATIVE HUMIDITY IS THE RATIO OF THE ACTUAL WATER CONTENT OF AIR TO THE MAXIMUM AMOUNT OF WATER THAT CAN BE HELD IN AIR AT A GIVEN TEMPERATURE.

 

   THE HIGHER THE RELATIVE

   HUMIDITY THE HIGHER THE

   VAPOR PRESSURE

 

   AIR THAT IS SATURATED (100%

   RH) HAS TWICE THE VAPOR

   PRESSURE AS AIR THAT IS ONLY

   50% SATURATED.

 

   THE VAPOR PRESSURE OF THE SUB-

   STOMATAL CRYPT IS CLOSE TO

   SATURATION WHILE AIR OUTSIDE    THE LEAF IS USUALLY NOT 100%    SATURATED SO THE VAPOR    PRESSURE IS GREATER INSIDE THE    LEAF

 

   THE CLOSER THE RH OUTSIDE THE

   LEAF IS TO 100% THE SHALLOWER

   THE VP GRADIENT AND THE

   SLOWER THE RATE OF TRANS-

   PIRATION.

 

EFFECT OF TEMPERATURE

 

AS TEMPERATURE INCREASES, THE

ATMOSPHERE IS ABLE TO HOLD MORE

WATER.  IF NO NEW WATER IS ADDED

TO THE SYSTEM, RH WILL DROP

 

 

 

 

         LEAF    ATMOSPHERE

TEMP.    100      100

RH       100%       50%

 

TEMP     200      200

RH       100%       26%

 

TEMPERATURE AFFECTS THE VP

GRADIENT

 

EFFECT OF WIND

 

WIND HAS AN AFFECT ON BOUNDARY LAYER THICKNESS.  THE HIGHER THE WIND SPEED THE THINNER THE BOUNDARY LAYER.

 

   THE THINNER THE BOUNDARY LAYER

   THE GREATER THE VAPOR

   PRESSURE GRADIENT AND THE

   HIGHER THE RATE OF    TRANSPIRATION